In the drilling of oil and gas wells, a drill bit is typically located at the end of a drill string. Rotation of the drill string causes the drill bit to drill into the well. It may be necessary in certain situations to deviate from drilling the well in a vertical direction. This is referred to as directional drilling. A known method of directional drilling includes the use of a rotary steerable system. In such systems, down hole devices steer the drill bit in a desired direction.
The rotary steerable system can include a bias unit that is located adjacent to the drill bit. The bias unit applies force to the drill bit in a controlled direction while the drill string rotates. The bias unit includes a plurality of bias pads that are actuated by drilling fluid or mud through a valve. The valve actuates the bias pads by sequentially diverting the drilling fluid into the piston chamber of each pad. Each bias pad is movable between a retracted position and an extended position in which the bias pad is positioned against the wall of the well. The drill bit can be urged in a desired direction by controlling the movement of the bias pads.
The manufacture and maintenance of conventional valves may be complex. Conventional valves generally limit the rotational speed of the drill bit to a maximum rotational speed of 200 RPM due in part to limited backflow capability of conventional valves.